Black phosphorus quantum dot-based field-effect transistors with ambipolar characteristics

Accepted Manuscript Full Length Article Black Phosphorus Quantum Dot-based Field-effect Transistors with Ambipolar Characteristics Soonjoo Seo, Byoungnam Park, Youngjun Kim, Hyun Uk Lee, Hyeran Kim, Seung youb Lee, Yooseok Kim, Jonghan Won, Youn Jung Kim, Jouhahn Lee PII: DOI: Reference:

S0169-4332(18)31120-6 https://doi.org/10.1016/j.apsusc.2018.04.158 APSUSC 39157

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

23 January 2018 3 April 2018 16 April 2018

Please cite this article as: S. Seo, B. Park, Y. Kim, H. Uk Lee, H. Kim, S. youb Lee, Y. Kim, J. Won, Y. Jung Kim, J. Lee, Black Phosphorus Quantum Dot-based Field-effect Transistors with Ambipolar Characteristics, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.04.158

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Black Phosphorus Quantum Dot-based Field-effect Transistors with Ambipolar Characteristics Soonjoo Seoa, Byoungnam Parkb, Youngjun Kimb, Hyun Uk Leea,*, Hyeran Kima, Seung youb Leea, Yooseok Kima, Jonghan Wona, Youn Jung Kimc and Jouhahn Leea,* a

Advanced Nano-Surface Research Group, Korea Basic Science Institute (KBSI), Daejeon

34133, Republic of Korea b

Department of Materials Science and Engineering, Hongik University, Seoul, 04066,

Republic of Korea c

Center for Research Facilities, Andong National University, Andong 36729, Republic of

Korea

Semiconductor quantum dots have intriguing electronic and optical properties distinguished from bulk owing to quantum confinement effects. Among the two-dimensional materials, black phosphorus (BP) has generated enormous excitement due to its tunable direct band gap and high p-type semiconducting properties. We prepared BP quantum dots (BPQDs) by simple liquid exfoliation using distilled water and ethanol solution. Our structural data show the uniform distribution of circular BPQDs with the average lateral size of 4.08  0.66 nm and the height of 1.130.32 nm. We fabricated BPQD field-effect transistors (FETs) to investigate the electrical characteristics of BPQD-based devices and found that both hole and electron transport can be probed in the BPQD FETs. The BPQD FETs exhibited unprecedentedly ambipolar behavior with the mobility of 0.11 cm2V-1s-1 for p type and 0.09 1

cm2V-1s-1 for n type at 300 K. Our results provide the simple preparation methods to fabricate ambipolar BPQD FETs with the comparable hole and electron transport for large-area applications in solar cells and optoelectronic devices.

*E-mail: [email protected], [email protected]

1. Introduction Semiconductor quantum dots (QDs) promise a great potential in device applications due to their unique optical and electronic properties [1-3]. A quantum dot is a semiconductor nanocrystal in which excitons are confined in all three special dimensions. Quantum dots show a wide range of size-dependent properties in contrast to their bulk counterpart [4]. Graphene quantum dots (GQD), for example, are small graphene fragments exhibiting quantum confinement effects, which results in a non-zero band gap [2,3] and luminescence on excitons [5,6]. The bandgap can be tuned by changing the size of GQDs [7,8]. In addition, the size-dependence on the optical absorption makes QDs more desirable than bulk or thin films for a variety of applications such as biological agents, photovoltaics and optoelectronic devices [9-11]. Among two-dimensional (2D) materials such as graphene and transition-metal dichalcogenides (TMDs), black phosphorus (BP) has triggered a surge of research interest as BP has a tunable and thickness-dependent band gap between 0.3 and 2 eV [12,13]. In addition, BP exhibits a high carrier mobility of ~ 104 cm2/Vs with on/off ratio of 104-105 [12,14]. Owing to the unique electronic and optical properties, BP has been strong candidates for applications in field-effect transistors (FETs) [15-18], gas sensors [19] and solar cells [15].

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Previous work reported that BP FETs were primarily involved with p-type conduction [12,20]. According to the band structure calculations, comparable FETs can be realized because of the similar effective masses of conduction and the valence band [20]. Recent publications show the ambipolar characteristics of BP FETs [13,21,22]. The intriguing properties of BP lead to highly anisotropic electrical conductivities and optical responses [14,23]. High hole mobility of 190 - 1000 cm2/Vs [20,24] and electron mobility of 20 - 89 cm2/Vs have been demonstrated with BP FETs [21,25]. Most of the device performances using BP have been demonstrated on few layered BP and ultrathin BP films using mechanical exfoliation [15-17]. Although mechanical exfoliation is commonly employed to cleave bulk BP to mono or few layers of BP, the method is not appropriate for large area applications. On the other hand, liquid exfoliation method provides a simple preparation process and thus suits for large-area applications in electronic devices [26-28]. For future applications in nanodevices, it is more promising to convert from nanosheets to QDs. Recently, liquid exfoliation has been used to produce BP quantum dots (BPQDs) using 1-methyl-2-pyrro-lidone (NMP) and DMSO and successfully employed for applications in memory devices, photothermal agents and sensors [29-31]. The BPQDs exhibit nonvolatile rewritable memory effect, near-infrared light photoexcitation for cell death, and cell tracking capability. However, the electrical properties of BPQDs in FETs has not been explored yet, which is important for electronic and photonic applications in photovoltaics and photodetectors. We report the electrical characteristics of BPQD FETs using a facile liquid exfoliation technique for the preparation of BPQDs dispersed in distilled water (DI) and ethanol solution. Since bulk BP consists of puckered layers stacked together by weak van der 3

Waals interactions, the mechanical exfoliation method has been used to prepare few-layer BP nanosheets [12,20]. Although mechanical exfoliation is commonly employed to cleave bulk BP to mono or few layers of BP, the method is not appropriate for large area applications. The liquid exfoliation provides much simpler synthesis method than the conventional method found in previous work and proved to be eco-friendly, biocompatible, and cost-effective [32]. A simple synthesis of BP nanosheets or thin films by liquid-based exfoliation have been previously reported [27,28]. The structural analyses were performed by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The electrical characteristics of BPQDs demonstrate that BPQD FETs can be switched from p type to n type.

2. Experimental 2.1 Sample Preparation Black phosphorous (0.4 g, 12.8 mmol) was dispersed in DI water (50 mL) and ethanol (50 mL) solution using high-intensity ultrasound irradiation for 30 min to form few layers of BPQDs. The supernatant liquid (10 mL) from the dispersed solution was dissolved in 100 mL of ethanol solution, followed by ultrasound-irradiation for 20 min. Two cycles of this procedure were performed to obtain BPQDs. The BPQDs were dispersed in DI water (20 mL) and ethanol (20 mL) solution with high-intensity ultrasonic energy at 2 0 kHz in a polypropylene bottle reactor using a Sonics and Materials VC750 ultrasoni c generator with a 100 W electrical energy.

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2.2 Structural Characterization The XPS spectra were collected using a Kratos Axis Ultra DLD spectrometer. The source was monochromatic Al Kα radiation (1486.6 eV) operated at 150 W.

The survey

scan and the core-level spectra were collected at a normal emission using 1.0 and 0.05 eV step sizes, respectively. For data acquisition, pass energies of 160 and 40 eV were used for survey and high-resolution core-level scans, respectively. The binding energy was referred to the Au 4f by Au disk in the electrical contact with the sample. The crystal structure of BPQDs was analyzed using X-ray diffractometer (Rigaku RDA-cA, Japan) by Cu K radiation through a nickel filter. The morphology data of the BP quantum dots were collected by AFM (Digital Instruments Multimode IV, USA), SEM (LEO 1530, Carl Zeiss, Germany), and high-resolution transmission electron microscopy (HR-TEM; JEOL, JEM 2200, Japan). For the AFM analysis, BP solution was spin-coated at 2500 rpm for 30s twice on a SiO2/Si (001) wafer and dried in an oven at 70 C overnight. For SEM measurements, the same samples for the AFM analysis were used with the accelerating voltage from 100 V to 1 kV. Raman spectroscopy measurements were performed (Renishaw, RM1000-Invia) in a backscattering configuration excited with a visible laser light (wavelength=514 nm), a notch filter cut-off frequency of 50 cm-1 and a focus spot size of 5 m. The spectra were collected through a 100 objective lens and recorded on a 188 lines mm-1 grating providing a spectral resolution of ~ 1 cm-1. The power levels were kept at 0.1 mW and the integration time was maintained for 5 s during the measurements to prevent samples from laser induced heating and ablation.

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2.3 Electrical Characterization BP dispersed in DI water was drop-casted on an FET device in which the source (Au (80 nm)/Ti (5 nm)) and the drain electrodes are pre-patterned onto the 200-nm thick SiO2 gate dielectric substrate using photolithography. The channel length and the width are 20 m and 2 mm, respectively. Highly p-doped Si substrate serves as a gate electrode. For the electrical characteristics of BPQDs, the BPQD solution was spin-coated at 1500 rpm for 60 s twice and dried in an oven at 70 C overnight.

3. Results and Discussion The Scheme 1 illustrates our liquid exfoliation method that uses sonication energies to cleave bulk BP to BPQDs in DI water and ethanol solution. This method allows us to exfoliate bulk BP into few layers of BP and the further process using ultra-sonication breaks up the van der Waals forces of the BP layers to form BPQDs. The two photos in Scheme 1 show bulk BP and dispersed BPQDs in solution. The crystal structure of BPQDs was investigated using XRD (Rigaku RDA-cA X-ray diffractometer, Japan). The XRD pattern of the BPQDs as shown in Fig. 1 (a) reveals the reflective peaks at 2 = 16.9, 34.2 and 52. 3 with the interplanar distance of d020= 5.2 Å, d040= 2.6 Å and d060=1.7 Å, respectively. The XRD results indicate that the BPQDs are crystalline and orthorhombic, which agrees with the crystal structure of bulk BP [33, 34]. BPQDs were characterized by Raman and compared with bulk BP as shown in Fig. 1 (b). In all samples, one out-of-plane phonon modes A1g and two in-plane modes B2g and A2g were observed. The three peaks of BPQDs were detected at 362.2 cm-1 for A1g, 439.3 cm-1 for B2g, and 466.1 cm-1 for A2g. Our Raman results are consistent with BPQD values of previous 6

studies [29,31]. In comparison to bulk BP, A1 g, B2g, and A2 g modes for BPQDs were redshifted by 5.9, 9.2, and 9.8 cm-1, respectively. The frequency difference between A1g and B2g modes changed from 73.7 cm-1 for bulk BP to 77.0 cm-1 for QDs. A similar red shift phenomenon has been reported elsewhere as summarized in Table 1 [29]. Previous work proved that the frequency values of all three modes are increased with decreasing number of layers of BP [35]. Hence, these Raman results clearly indicate that BPQDs are distinguished from bulk BP with in terms of the phonon mode frequencies and the thickness of the BP layers. The chemical composition of BPQDs was analyzed using XPS. Fig. 1(c) and Fig. 1 (d) show a survey scan and the decomposition result of P 2p core-level spectra taken from the BPQD sample on a gold disk. The survey scan in Fig. 1(c) shows the dominant peaks assigned for oxygen (O 1s and O KLL), carbon (C 1s) and BPQDs (P 2p). A small amount of tin (Sn) is attributed to the fabrication process of BP single crystal. The decomposition of P 2p core-level spectrum into three components in Fig. 1 (d) was carried out using a standard nonlinear-least-squares fitting procedure with the Voigt function. The spin-orbit splitting of the P 2p3/2 and P 2p1/2 pair was observed at 130.0 eV and 130.86 eV, respectively and the branching ratio was 0.48. The binding energy of P 2p3/2 at 130.0 eV results from P-P bonds of BP which agrees well with the XPS results in previous studies [36,37]. The core-level shifts to higher binding energies by 4.3 eV (red spectrum) and by 5.1 eV (blue spectrum) are originated from phosphorus pentoxide (P2O5) and +5 oxidation states with a bond order 5, respectively [36,38]. XPS results clearly indicate that no chemical contamination is detected in our BPQD samples.

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The morphology of BPQDs was examined using AFM, SEM and TEM. The AFM images of BPQDs in Fig. 2 (a) and (b) indicate that the size and the distribution of BPQDs are uniform. The sectional profiles for the line segments A and B in the enlarged AFM image of BPQD are shown in Fig. 2 (c). The measured heights of BPQDs were 1.34 nm and 1.29 nm for Line A and 1.72 nm and 1.48 nm for Line B, which correspond to approximately 3 layers of BP. The surface morphology and the size of BPQDs measured by SEM in Fig. 2 (d) are almost the same as those of the AFM image. It was difficult to obtain high-resolution SEM images because the sample damage was so severe that BPQDs disappeared quickly. Although the low acceleration voltages between 100 V and 1 kV were used, BPQDs were hard to image due to the electron beam or carbon contamination problems. Since the penetration power by the beam into the sample is greater with a higher accelerating voltage in SEM, the possibility of specimen damage will increase in thin materials. The density of the BPQDs can be controlled by varying the rpm of spin coating. The size of BPQDs with different concentrations was similar. The TEM images in Fig. 3 (a) and 3 (b) show nano-sized BPQDs, which coincides with the AFM and SEM images. The high-resolution (HRTEM) images in Fig. 3 (c) and 3 (d) exhibit the lattice fringes of 0.20 nm and 0.19 nm which can be ascribed to (022) plane of a BP crystal. The statistical analysis reveals that the average lateral size of BPQDs measured from the TEM images is 4.080.66 nm (Fig. 3 (e)) and the measured average height from the AFM images is 1.130.32 nm (Fig. 3 (f)). The size to height ratio of BPQDs is about 4:1, which means the shape of BPQDs resembles a disk. The electrical characteristics of BPQDs were analyzed using bottom-contact FETs. BPQD solution was drop-casted between the source and the drain contacts on an FET device 8

as illustrated in Fig. 4 (a). Since it is important to cover the whole substrate with BPQDs to obtain an electrical conduction, BPQD solution was spin-coated at slower rpm for a longer time than those used for structural analyses. The AFM image in Fig. 4 (b) was obtained from a BPQD sample spin-coated at 1500 rpm for 60 s twice while the AFM images in Fig. 2 (a) and 2 (b) were obtained with 2500 rpm and 30 s. The slower spin coating rate and the longer time resulted in the higher density of BPQDs than those of AFM, SEM, and TEM images in Fig. 2 and Fig. 3. The electrical conduction was not detected for BPQD samples spin-coated at 2500 rpm for 30 s. In Fig. 4 (c), the FET device was operated in the linear transport regime of transistor operation. In the linear regime operation, a far smaller drain voltage, VD, than the gate voltage, VG, was applied to the channel between the source and drain electrodes. The FET hole mobility, μ, and the threshold voltage, VT, are given by eq. (1) in the linear transport regime:

ID 

Z Cox (VG  VT )VD L

(1)

where Cox is the capacitance of the gate dielectric. In the operation regime, the drain current increased linearly with the increasing gate voltage. The FET mobility is proportional to the slope in the linear region in the plot of ID vs. VG. The threshold voltage is defined as the minimum gate voltage to induce mobile charge carriers in the channel region. Above the threshold voltage, the drain current increases linearly with the increasing gate voltage, which satisfies the relation given in eq. (1). The threshold voltage is extracted from the gate voltage at which the drain voltage is zero in the linear regime. In Fig. 4 (c), ambipolar transport was measured in a BPQD FET. As the positive gate voltage increases at a small drain voltage of 1 V, the drain current increased due to the electron conduction in BPQDs, exhibiting typical n-channel FET characteristics. Similarly, in 9

the negative gate voltage region, the current increases with increasing negative gate voltages due to hole conduction forming a p-channel FET. The transfer characteristics of BPQDs clearly shows a pronounced ambipolar behavior. No studies of ambipolar characteristics of BPQDs exist in the literature. The threshold voltage for electron conduction was -14 V while the threshold voltage for hole conduction is 6.4 V, clarifying that the current at zero gate voltage is the sum of the electron and hole current. At zero gate voltage in Fig. 4 (d), as the drain voltage increases in the positive drain voltage region, the contribution of the hole current becomes more significant. The FET electron and hole mobility extracted from the linear regime of the transistor operation were 0.09 and 0.11cm2/Vs, respectively. Previous studies on BP FETs with ambipolar characteristics showed antisymmetric conduction; that is, the hole transport is dominant. Buscema et al. demonstrated that the FETs of mechanically-exfoliated BP have the mobility in the order of 100 cm2/Vs for hole transport and 0.5 cm2/Vs for electron transport [15]. In other work, the hole mobility ranges from 83 to 984 cm2/Vs and the electron mobility varies from 0.5 to 89 cm2/Vs. In comparison to bulk or few-layer BP, the mobility of BPQDs is much lower, which can be attributed to the intrinsic and extrinsic factors. Unlike mechanically exfoliated BP, BPQDs by liquid exfoliation feature a high surface to volume ratio, which leads to a high density of surface traps and localized carriers. A significant amount of BPQDs/gate dielectric interfacial traps can be created due to poor wetting of BPQDs on the SiO2 surface without any chemical treatment. Indeed, BPQDs exhibited small-sized island growth on the SiO2 surface at a very low coverage. The interface traps have significant effects on field effect-modulated charge transport in BPQD-based FETs because the effective accumulation length scale is few-layers deep next to the gate dielectric. The poor interfacial contact results in localized states at the interface, 10

serving as carrier scattering centers. The FET mobility can, therefore, be significantly degraded in the structurally and energetically disordered interface. The development of surface modification is necessary to reduce the gate dielectric/BP interfacial energetic and structural disorder. For example, the device performance of hybrid perovskite solar cells can be improved by spin-coating BPQDS onto PEDOT:PSS due to hole extraction effect and well-matched band alignment [39]. The size of QDs also has an influence on charge transport [40]. The synthesis of monodisperse BPQDs is crucial to achieve highly efficient charge transport because energy barrier for carrier transport can be formed with a wide range of size variation of BPQDs. Previous study reported that the size distribution can increase the “off” current and carrier mobility is influenced by the QD size [40]. The on/off ratio of BPQDs is small compared with that of bulk or 2D BP [12]. At zero gate voltage, the FETs were already turned on for electron and hole conduction, which increases the “off” current. We attribute the large off current to ionic conduction of mobile H + ions produced in the absorbed moisture layer via autoionization process of water molecules in the BPQD layer [41]. Indeed, the field effect modulation was decreased with a number of electrical measurements because the migration of ions during the application of gate electric field altered the electron and hole concentrations, which resulted in shifting the threshold voltage to a more positive or more negative value. This explains the instability of threshold voltage after the fabrication of FET devices. Additionally, the large metal/BPQDs contact resistance, as plotted in the I-V curve in Fig. 4 (d), can decrease the FET mobility. One method to improve the stability of BPQDs is the elimination of intrinsic and oxidation defects. Oxygen induces charge traps and scattering centers in BPQD near SiO 2, 11

reducing the carrier mobility. It is known that the intrinsic point defects and oxidation giving rise to in-gap-states are unavoidable during the synthesis process of BPQDs, which results in atmospheric degradation [29, 30, 32]. The in-gap-states could be removed by hydrogen passivation [42]. The doping or capping technique of 2D materials can minimize the atmospheric degradation and further improve the device performance. For instance, Fang et al. reported that potassium-doped MoS2 and WSe2 enhanced electron charge transfer and thus exhibited the electron mobility comparable to the hole mobility [43]. The significant reduction of contact resistance was achieved in chloride-doped MoS2 and WS2 [44]. In addition, the encapsulation of ultra-thin BP FETs with hexagonal boron nitride can make extremely robust devices to environment [17].

4. Conclusions We have successfully synthesized BPQDs by liquid exfoliation method using DI water and ethanol solution. The BPQDs have the lateral size of 4.08 nm0.66 nm and the height of 1.130.32 nm. Our results clearly show that BPQD FETs exhibit ambipolar behavior with the comparable hole and electron mobility. The lower mobility values of BPQDs than that of bulk BP is presumably due to both intrinsic and extrinsic effects. Particularly, for nano-sized BPQDs, surface traps and size variation can significantly affect charge transport. The optimization of fabrication conditions is required to improve charge transport and to minimize extrinsic effects such as ionic conduction, wide size distribution, and poor wetting on the gate dielectric. The further development of doping and capping techniques including surface modification is necessary to reduce the gate dielectric/BP interfacial energetic and structural disorder as well as atmospheric degradation. 12

Acknowledgements This research was supported by the Korean Basic Science Institute research Grant No. C38116. Also, the work partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. 2015R1A6A1A03031833).

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Table 1. Comparison of prominent Raman peaks for three phonon modes for bulk BP and BPQDs. The superscripts a, b, c, d, e, f, and g indicate BPQD from Ref. 29, BPQD from Ref. 31, BPQD of our sample I, BPQD of our sample II, bulk BP from Ref. 29, bulk BP from Ref. 31, and bulk BP from our data, respectively. The number in each parenthesis means the phonon mode shift in cm-1. 361.6a (5.6) 359.5b (1.3)

361.6c (5.3)

362.2d (5.9)

356.0e

358.2f

356.3g

438.7a (8.9)

436.0b (2.4)

437.9c (7.9)

439.2d (9.2)

429.8e

433.6f

430.0g

466.1a (8.9)

463.3b (1.7)

465.3c (8.5)

466.6d (9.8)

457.2e

461.6f

456.8g

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Figure Captions Scheme 1. Schematic diagram of the disintegration process from bulk BP to BPQDs using ultrasonic energy.

Fig. 1. (a) XRD results of BPQDs and bulk BP. (b) Raman spectrum of BPQDs. (c) XPS survey spectrum of the BPQDs. (d) High-resolution P 2p spectrum of BPQDs.

Fig. 2. (a) AFM images of BPQDs. (b) Larger view of BPQDs. (c) Height profiles along the Line A and Line B in (b). (d) SEM image of BPQDs.

Fig. 3. Typical TEM bright-field images (a) and (b) TEM images of BPQDs. (c) and (d) High-resolution TEM images of BPQDs. (e) Statistical analysis of the lateral size of BPQDs measured from TEM images. (f) Height measurements of BPQDs obtained from AFM images.

Fig. 4. (a) Schematic of the device structure of BPQD FET. (b) AFM image of the BPQD sample used for the electrical characterization of the BPQD FET. (c) Transfer characteristics (Id vs Vg) of BPQD FET. (d) I-V curves of BPQD FET.

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Scheme 1

22

Fig. 1

23

Fig. 2

24

Fig. 3

25

Fig. 4

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Highlights

► The BPQDs can be mass-produced at room temperature. ► The BPQDs have the lateral size of ~4.0 nm and the height of ~1 nm. ► The BPQDs FETs exhibits ambipolar behavior with the comparable hole and electron mobility.

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BP quantum dot-based field effect transistor and its characteristic behavior.

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